On-board motor vehicle electrical systems having a DC/DC converter with a high-voltage side and a low-voltage side, with at least one traction battery being connected to the high-voltage side and electrical loads being connected to the low-voltage side, are used, in particular, in hybrid or electric vehicles. However, in principle, arrangements of this kind can also be used in motor vehicles, with drive power being provided, for example, only by means of an internal combustion engine. In these cases too, the battery on the high-voltage side is currently called the traction battery. In the case of an on-board motor vehicle electrical system, the voltage on the low-voltage side is, for example, 12 V, while the voltage on the high-voltage side can be, for example, in the region of 300 V, 600 V or 750 V depending on the design of the system. In this case, the voltage level depends on the cell voltage of the battery cells used and the number of battery cells which are connected in series. The following applies for a predetermined drive power: the higher the output voltage of the traction battery is selected to be, the lower are the currents required to achieve the selected drive power. With lower currents, the cross sections of the electrical connections can be selected to be lower given the same or else similar line resistances. This generally leads to relatively low weight. A traction battery is a battery which stores energy for driving the motor vehicle and provides this energy during operation. In the case of a hybrid vehicle, this drive energy of the traction battery can also be used to assist driving, so that a further drive, for example a motorized drive, is available in parallel.
An on-board motor vehicle electrical system of this generic type is known, for example, from DE 10 2007 052 750 A1. Furthermore, the document discloses a method for starting a hybrid drive by means of an auxiliary energy source comprising the steps of: providing an electrical machine and an internal combustion engine which together are intended to generate kinetic energy in the hybrid drive; providing a traction battery for supplying traction energy to the electrical machine, an on-board electrical system for starting the internal combustion engine in the normal mode, and a converter control system of a DC/DC converter which is provided for supplying the on-board electrical system from the traction battery, with the on-board electrical system supplying the converter control system. If, in an emergency mode on account of an energy deficit in the on-board electrical system, there is not enough energy remaining in the on-board electrical system either to operate the converter control system or to start the internal combustion engine, an external auxiliary amount of energy is supplied to the converter control system, this auxiliary amount of energy being lower than the energy deficit, in order to at least briefly ensure operation of the converter control system; activating the DC/DC converter by actuation by means of the converter control system and feeding an amount of energy for a starting process, which originates from the traction battery, to the on-board electrical system by the DC/DC converter, this amount of energy, together with the energy remaining in the on-board electrical system, being sufficient to start the internal combustion engine, with the internal combustion engine being started by the transmission of rotational energy from the starter.
The known on-board motor vehicle electrical systems have a significant weight and installation space requirement on account of the traction battery, the DC/DC converter and the on-board electrical system battery on the low-voltage side which serves to supply the controllers.